A unique and known electronic shell structure is associated with each atomic species of matter. The electrons populating the shell structure are associated with energies and levels of excitation described by the laws of quantum mechanics. It is known that an electronic transition from a first shell or energy state to a second shell or energy state is accompanied by a discrete emission of energetic radiation, such as photons. These radiative emissions are sometimes termed “atomic” to indicate that they correspond to atomic shell structure transitions, and include gamma rays, X-rays, visible light, and invisible photonic radiation. Other radiative emissions, sometimes termed “nuclear” emissions are due to interactions and transitions occurring within the nucleus of an atom rather than in the electronic shell structure.
It is also known that atoms may interact with or capture energetic particles, causing the atoms to become excited, forming “exotic atoms.” An exotic atom is generally one which has captured a negatively-charged subatomic particle into its electronic shell structure, and can be accompanied by complex physical behavior within the electronic shell structure. Examples of subatomic particles which may cause an atom to become an exotic atom include negative muons and pions. An exotic atom may then return to its ground state by emission of one or more energetic radiative emissions, such as X-rays, having known characteristic and unique energies.
This behavior has been exploited in the past for antiparticle detection and identification. Antimatter particles were captured by one or more nitrogen atoms to create exotic nitrogen atoms, which subsequently decayed to a lower energy state. The deexcitation of a nitrogen atom released characteristic subatomic particles, which were detected using the segmented X-ray spectrometer and used to identify the antimatter particle, e.g. an antiproton. The antimatter particle was then identified by studying the unique fingerprint provided by the characteristic emitted particles as the nitrogen atom decayed from its excited exotic state to its ground state. This method is inherently fairly immune to noise, since detection of the unique characteristic X-ray spectrum is a sure indication of the type of antimatter particle that excited the nitrogen atoms.
Another application involves the use of muons in radiography. Muons are negatively-charged leptons, or “heavy electrons,” known to have a small cross section for absorption in matter, and are therefore capable of penetrating materials which would otherwise be relatively opaque to traditional ionizing radiation. Because muons are deflected when penetrating dense or heavy materials rather than being absorbed, they can be used to detect the presence of such dense materials hidden within containment devices or shielded from traditional detection.
It has been suggested that a source of muons and a detector may be placed on either side of a container holding an unknown material. By using the detector to detect the deflection of the muons, it can be surmised whether dense materials are present in the container. In one application, it has been suggested that this technique could be used to detect banned materials, such as nuclear materials, hidden in shipping containers or suitcases. Others in the field have suggested that an artificial source of muons or a natural source of muons can be used for such purposes.
Subatomic particles can be generated in a laboratory or can be naturally-occurring. Artificially-generated muons are produced in accelerators, which can be room-sized according to the present state of the art. Naturally-occurring muons are created in atmospheric interactions with cosmic radiation, and are therefore sometimes called “secondary cosmic radiation.”